The present invention generally relates to track retrieval systems, and more particularly to a track retrieval system for retrieving a track of an optical disk by counting track cross pulses.
An optical disk unit controls access to a track of an optical disk by generating a track cross pulse every time a scan beam moves 1 track or 1/2 track, for example, and counting the track cross pulses. The track cross pulses are generated based on a track error signal which is dependent on a tracking error of the scan beam relative to the track. For this reason, a chattering easily occurs in the track cross pulses. 0n the other hand, the track cross pulse easily drops out when the scan beam traverses an address part of the track.
FIG. 1 shows an example of a conventional track retrieval system. In FIG. 1, an optical disk 51 has concentric tracks or a spiral track formed thereon. An optical pickup 52 is driven by a voice coil motor (VCM) 53 and is moved in a radial direction of the optical disk 51. The optical pickup 52 irradiates a scan beam on the optical disk 51 so as to write/read information to/from the optical disk 51.
For example, the optical disk 51 is in conformance with the standards set by the International Organization for Standardization (ISO) for 130 mm (5-inch) optical disks or, for 90 mm (3.5-inch) optical disks.
A tracking error of the scan beam relative to the track of the optical disk 51 is detected based on the reflected beam from the optical disk 51. A track error signal which indicates this tracking error is output from the optical pickup 52 and is formed into a pulse signal by a pulse shaping circuit 54. Pulses of the pulse signal output from the pulse shaping circuit 54 is counted by a counter circuit 55 as track cross pulses. A microprocessor unit (MPU) 56 controls the VCM 53 via a digital-to-analog (D/A) converter 57 and a power amplifier 58 based on the counted value of the counter circuit 55.
FIG. 2 is a time chart for explaining the operation of the track retrieval system shown in FIG. 1. In FIG. 2, (A) shows the track error signal output from the optical pickup 52, (B) shows the track cross pulses output from the pulse shaping circuit 54, (C) shows a target speed within the MPU 56, (D) shows a calculated speed within the MPU 56, and (E) shows a control signal supplied to the VCM 53.
The counter circuit 55 counts the track cross pulses shown in FIG. 2 (B). The MPU 56 loads a number of tracks to a target track into the counter circuit 55, and the counter circuit 55 subtracts the number of track cross pulses from the loaded number of tracks. The MPU 56 reads the output of the counter circuit 55 at predetermined intervals, and calculates the moving speed of the optical pickup 52 by calculating the position of the optical pickup 52. In addition, the MPU 56 compares the calculated speed and the target speed, and calculates a control quantity of the VCM 53 depending on the position of the optical pickup 52. The control signal shown in FIG. 2 (E) is supplied to the VCM 53 as this control quantity.
FIG. 3 is a flow chart for explaining the above described operation of the MPU 56. In FIG. 3, a step S1 presets a moving quantity N.sub.M of the optical pickup 52 into the counter circuit 55. This moving quantity N.sub.M is preset from a host controller (not shown) via the MPU 56. Then, a step S2 initializes a counted value N.sub.T-1 of the counter circuit 55 to N.sub.M, and a step $3 waits for a time .DELTA.T.
A step S4 reads the counted value N.sub.T of the counter circuit 55 corresponding to the distance to the target position. A step S5 calculates a moving speed V of the optical pickup 52 based on V=N.sub.T-1 -N.sub.T. A step S6 calculates a target speed V.sub.M based on V.sub.M =F(N.sub.T), where V.sub.M =F(N.sub.T), is stored in a memory (not shown) which is connected to the MPU 56.
A step S7 calculates a speed error .DELTA.V based on .DELTA.V=V-V.sub.M, and a step S8 calculates a current I.sub.V to be supplied to the VCM 53 based on I.sub.V =K.times..DELTA.V, where K is a predetermined constant. A step S9 outputs the current I.sub.V to the VCM 53 via the D/A converter 57 and the power amplifier 58. Finally, a step S10 updates the counted value N.sub.T-1 to N.sub.T, and the process returns to the step S3.
If one track cross pulse is generated every time the optical pickup 52 (that is, the scan beam) moves 1 track or 1/2 track, a chattering occurs in which a plurality of pulses are successively generated as the track error signal makes a zero-crossing or, a pulse dropout of the track cross pulses occurs when the scan beam scans the address part and passes a part where no grooves are formed on the optical disk 51.
The chattering occurs particularly when a comparator compares an analog signal having a relatively low frequency with another signal. On the other hand, the pulse dropout of the track cross pulse occurs particularly when the optical pickup 52 (that is, the scan beam) moves at a high speed. FIG. 4 is a time chart for explaining the chattering, and FIG. 5 is a time chart for explaining the pulse dropout of the track cross pulses. In each of FIGS. 4 and 5, (A) shows the track error signal and (B) shows the track cross pulses.
Next, a description will be given of a conventional method of preventing the chattering, by referring to FIG. 6.
FIG. 6 shows the pulse shaping circuit 54 which is designed to prevent the chattering. This pulse shaping circuit 54 includes so-called hysteresis comparators 61 and 62, D-type flip-flops 63 and 64, and a delay circuit 65.
The comparator 61 has a non-inverting input terminal for receiving a negative reference voltage -A and an inverting input terminal for receiving the track error signal. The comparator 62 has a non-inverting input terminal for receiving a positive reference voltage +A and an inverting input terminal for receiving the track error signal. An output signal of the comparator 61 is input to a clock terminal CLK of the flip-flop 63, and an output signal of the comparator 62 is input to a clock terminal CLK of the flip-flop 64.
An input terminal D of the flip-flop 64 is connected to a D.C. power source voltage V.sub.DC, and an output terminal Q of the flip-flop 64 is connected to an input terminal D of the flip-flop 63. A signal output from an output terminal Q of the flip-flop 63 is delayed by a predetermined time .DELTA.D in the delay circuit 65 before being input to clear terminals CLR of the flip-flops 63 and 64. Hence, track cross pulses prevented of the chattering are output from an output terminal Q of the flip-flop 63.
As shown in FIG.7, the pulse shaping circuit 54 shown in FIG. 6 sets two slice levels (reference voltages) +A and -A by the two comparators 61 and 62 having mutually different slice levels. The track cross pulse is generated when the track error signal exceeds one slice level -A after exceeding the other slice level +A.
On the other hand, as a previously proposed method of correcting the pulse dropout of the track cross pulses, there is a method which measures an immediately preceding pulse interval and sets a time based on the measured pulse interval. If no track cross pulse is generated within this set time, it is regarded that a pulse dropout has occurred and one track cross pulse is counted although no track cross pulse exists.
In addition, other methods have also been proposed. For example, a Japanese Laid-Open Patent Application No. 1-98169 proposes a method of correcting the chattering and the pulse dropout, and a Japanese Laid-Open Patent Application No. 63-10382 proposes a method of correcting the pulse dropout.
According to the method proposed in the Japanese Laid-Open Patent Application No. 1-98169, a prohibit circuit is provided. This prohibit circuit supplies a prohibit signal to the counter circuit 55, the MPU 56 and the like during a time in which the optical pickup 52 (scan beam) passes the address part of the optical disk 51 so as to prohibit the counting operation during this time. The MPU 56 calculates the number of tracks passed during the time in which the prohibit signal is output from the prohibit circuit, and directly corrects the counted value of the counter circuit 55 based on the calculated number of tracks.
On the other hand, the method proposed in the Japanese Laid-Open Patent Application No. 63-10382 only corrects the pulse dropout using an integrator. The integrator integrates an output signal of a speed detector which is connected to the VCM 53, and calculates a moved distance of the optical pickup 52 (scan beam) based on the integrated value. If no track cross pulse is generated even after the moved distance exceeds a predetermined distance (distance between tracks), a correction pulse is generated to correct the pulse dropout of the track cross pulse, and at the same time, the integrator is reset.
However, according to the pulse shaping circuit 54 shown in FIG. 6, it is difficult to form this circuit in the form of an integrated circuit (IC) because of the need to provide comparators, variable resistors and the like. As a result, there is a problem in that it is difficult to reduce the size of the pulse shaping circuit 54.
As for the previously proposed method which measures the immediately preceding pulse interval and sets the time based on the measured pulse interval, there is a problem in that it is difficult to set a time which covers a high-speed range in which the optical pickup 52 (scan beam) moves at a high speed of 3 .mu.s/track to a low-speed range in which the optical pickup (scan beam) moves at a low speed of 1 ms/track, for example. If the time were to be set to cover these ranges, it would require approximately 8000 gates to perform the required operations by hardware. Similarly, the same problem would occur if the chattering were to be prevented by setting the time which covers the high-speed range to the low-speed range of the optical pickup 52 (scan beam).
On the other hand, according to the method proposed in the Japanese Laid-Open Patent Application No. 1-98169, the scale of the circuit becomes large if a MPU is independently provided exclusively for making the correction. Further, if the MPU 56 which controls the VCM 53 is also used for making the correction, the MPU 56 must simultaneously process the motor control and the interrupt from the prohibit circuit and the control program of the MPU 56 becomes complex. In addition, a problem occurs if the predicted speed used for the correction is different from the actual speed of the optical pickup 53 (scan beam), because the corrected track cross pulses are input directly to the counter circuit 55. For example, if the actual speed is smaller than the predicted speed, the original track cross pulse is input immediately after one pulse is corrected within the time of the prohibit signal, and there is a problem in that an unnecessary correction is made.
With regard to the method proposed in the Japanese Laid-Open Patent Application No. 63-10382, the error of the speed detector tends to accumulate because the integrator is reset regardless of the track error signal. Moreover, there is a problem in that it is difficult to reduce the size of the circuit because of the need to provide the speed detector and the integrator, particularly since it is difficult to form the integrator in the form of the IC.